Sapphire substrate and method for manufacturing the same and nitride semiconductor light emitting element

ABSTRACT

A sapphire substrate provided with a plurality of projections on a principal surface on which a nitride semiconductor is grown to form a nitride semiconductor light emitting element. The projections have a substantially triangular pyramidal-shape the projections having a plurality of side surfaces and a pointed top. The side surfaces have an inclination angle of between 53° and 59° from a bottom of the projections. The side surfaces are crystal-growth suppressed surfaces on which a growth of the nitride semiconductor is suppressed relative to a portion of the principal surface located between adjacent projections. A bottom of the projections has a substantially triangular shape having three outwardly curved arc-shaped sides, and each of the side surfaces has a substantially triangular shape having vertexes located at the top of the projection and at both ends of a respective side of the bottom of the projection.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to JapanesePatent Application No. 2012-074667, filed Mar. 28, 2012, and JapanesePatent Application No. 2013-049919, filed Mar. 13, 2013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present Invention relates to a sapphire substrate for a nitridesemiconductor light emitting element and a method for manufacturing thesame and a nitride semiconductor light emitting element.

2. Description of Related Art

Generally, a light emitting diode (LED) made of a nitride semiconductoris constituted by laminating on a sapphire substrate an n-typesemiconductor layer, an active layer and a p-type semiconductor layer inthis order. Light generated in this light emitting diode at the activelayer is extracted from a side opposite to the sapphire substrate orfrom the sapphire substrate side, while the generated light is alsoradiated to a direction opposite to a light exiting side. Therefore, itis necessary to improve external quantum efficiency by allowing lightradiated to a direction opposite to the light exiting side to beextracted effectively from the light exiting side.

Therefore, JP 2008-177528A, for example, discloses that an externalquantum efficiency of a semiconductor light emitting element is improvedby arranging a plurality of projections having truncated triangularpyramidal-shape on a sapphire substrate. It also describes that thegeneration of voids and the deterioration of crystallinity can besuppressed by growing a nitride semiconductor crystalline on a surfaceon which truncated triangular pyramidal-shaped projections are formed.

However, with an increase in an output of light emitting diodes, it hasbeen found as a result of the present inventors' studies that problemsdue to crystal defects become apparent, which were not recognized in theprevious light emitting diodes. In addition, further improvements aredesired in a light extraction efficiency of light emitting diodes.

Thus, an object of the present invention is to provide a sapphiresubstrate and a method for manufacturing the same, which enables growthof a nitride semiconductor having excellent crystallinity and canprovide a nitride semiconductor light emitting element having excellentlight extraction efficiency.

Furthermore, an object of the present invention is to provide a nitridelight emitting element which comprises a nitride semiconductor havingexcellent crystallinity and light extraction efficiency.

SUMMARY OF THE INVENTION

One aspect of the present invention is a sapphire substrate providedwith a plurality of projections on a principal surface on which anitride semiconductor is grown to form a nitride semiconductor lightemitting element, wherein the projections have a substantiallypyramidal-shape constituted by a plurality of side surfaces to have apointed top, wherein the side surfaces have an inclined angle of between53° and 59° from a bottom surface of the projections, and wherein theside surfaces are crystal-growth-suppressed surfaces on which a growthof the nitride semiconductor is suppressed relative to a substratesurface located between the adjacent projections.

Another aspect of the present invention is a method for manufacturing asapphire substrate in which a plurality of projections are formed onC-plane of the sapphire substrate by etching, comprising: forming apatterned etching mask on C-plane of the sapphire substrate; etching thesapphire substrate until the projections are formed, wherein theprojections formed by the etching have a substantially pyramidal-shapeconstituted by a plurality of side surfaces to have a pointed top, theside surfaces having an inclined angle of between 53° and 59° from abottom surface of the projections; and removing the etching masks fromthe sapphire substrate.

Further aspect of the present invention is a nitride semiconductor lightemitting element formed by growing a nitride semiconductor on theprincipal surface of the sapphire substrate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A shows a plan view of an example of a previous projection, andFIG. 1B shows a cross-section view of the projection shown in FIG. 1Balong the line A-A′.

FIG. 2 is a cross-section view of a nitride semiconductor light emittingelement in an embodiment of the present invention.

FIG. 3A is a plan view of a projection in an embodiment of the presentinvention, and FIG. 3B shows a perspective view of the projection shownin FIG. 3A.

FIG. 4 is a plan view showing an example of projection arrangement in anitride semiconductor light emitting element of an embodiment.

FIG. 5 is a plan view showing another example of projection arrangementin a nitride semiconductor light emitting element of an embodiment.

FIGS. 6A-6D schematically show the respective steps of a method formanufacturing a sapphire substrate according to one embodiment of thepresent invention.

FIG. 7A is a SEM image from a top side of a sapphire substrate ofExamples, and FIG. 7B is a SEM image at a cross section of the sapphiresubstrate of Examples.

FIG. 8 is a SEM image from a top side of a sapphire substrate ofComparative Examples.

FIG. 9 is a graph showing a luminous flux ratio of a light emittingdevice of Example 1 to that of Comparative Example 1.

FIG. 10 is a graph showing a radiant flux ratio and a luminous fluxratio of a light emitting device of Example 2 to that of ComparativeExample 2.

FIG. 11 is a SEM image at a cross section of the projections ofComparative Example 3.

FIG. 12A is a top view of the nitride semiconductor light emittingelements of Example 3 and Comparative Example 3, FIG. 12B shows adirectional characteristics of the nitride semiconductor light emittingelements of Example 3 and Comparative Example 3 when the lightintensities were measured in a direction of φ=0°, and FIG. 12C shows adirectional characteristics of the nitride semiconductor light emittingelements of Example 3 and Comparative Example 3 when the lightintensities were measured in a direction of φ=90°.

DESCRIPTION OF EMBODIMENTS Detailed Description

Hereinafter, embodiments of the present invention are described withreference to the drawings. However, embodiments described below areaimed at embodying the present invention, and the present invention isnot limited to the embodiments. In particular, dimensions, materials,shapes relative arrangement and the like of the components describedbelow are not intended to limit the scope of the present invention onlyto them unless specifically described, but they are merely illustrativeexplanations. In addition, dimensions, positional relations and the likeof members shown in each drawing may be exaggerated in order to makeexplanations clear. Furthermore, each component constituting embodimentsof the present invention may be adapted to an aspect in which aplurality of components are constituted by the same member and theplurality of components are combined into one member, or on thecontrary, can be achieved by sharing a function of one member with aplurality of members.

FIGS. 1A and 1B show an example of previous projections (see JP2008-177528A). The projections shown in FIGS. 1A and 1B have asubstantially truncated triangular pyramidal-shape with a bottom surfacehaving substantially triangular shape, and have a flat surface 104 atthe top of the projection parallel to a plane positioned between theadjacent projections. Each of the sides 101, 102 and 103 has anoutwardly curved arc shape, respectively. The flat surface 104 at thetop is a crystal growth surface on which a nitride semiconductor can begrown. On the other hand, three side surfaces 101 k, 102 k and 103 k arecrystal-growth-suppressed surfaces on which the growth of nitridesemiconductor is suppressed, respectively. Using a sapphire substratehaving such projections periodically arranged thereon, a nitridesemiconductor having excellent crystallinity can be grown and a nitridesemiconductor light emitting element having excellent light extractionefficiency can be obtained.

The sapphire substrate having the above-mentioned previous projectionsperiodically arranged thereon can further improve the light extractionefficiency by increasing the heights of the projections. However, it hasbeen found as a result of the present inventors' studies that voids arelikely to be generated in the nitride semiconductor grown on thesubstrate when the heights of the previous projections are increased.

The present inventors have intensively studied and found that bothimproved light extraction efficiency and excellent crystallinity can beachieved by using a sapphire substrate having a plurality ofsubstantially pyramidal-shaped projections constituted by side surfacesthat are inclined from bottom surfaces of the projections by 53° to 59°to have a pointed top, and thus, the embodiment has been completed.

The embodiment relates to a sapphire substrate provided with a pluralityof projections on a principal surface on which a nitride semiconductoris grown to form a nitride semiconductor light emitting element, whereinthe projections have a substantially pyramidal-shape constituted by aplurality of side surfaces to have a pointed top, wherein the sidesurfaces have an inclined angle of between 53° and 59° from a bottomsurface of the projections, and wherein the side surfaces arecrystal-growth-suppressed surface on which a growth of the nitridesemiconductor is suppressed relative to the substrate surface locatedbetween the adjacent projections.

The bottom surface of the projections preferably has a substantiallypolygonal shape having three or more outwardly curved arc-shaped sides,respectively, and each of the side surfaces preferably has asubstantially triangular shape of which vertexes are both ends of a sideof the bottom surface and the top of the projection. The bottom surfaceof the projection more preferably has substantially triangular shape.

The projections preferably have a height of between 1.0 and 1.7 μm.

The projections are preferably arranged to be apart from each other onthe principal surface of the sapphire substrate. Further, theprojections are preferably arranged periodically on the principalsurface of the sapphire substrate, and more preferably arranged on eachvertex of triangular, tetragonal or hexagonal lattice.

A distance between tops of the adjacent projections is preferablybetween 2.2 μm and 3.1 μm, more preferably between 2.8 μm and 3.1 μm.

The substrate surface located between the adjacent projections a crystalgrowth surface, and a ratio of an area of the crystal growth surface tothat of the principal surface is preferably between 25% and 60%, andmore preferably between 30% and 45%.

Furthermore, the embodiment relates to a method for manufacturing asapphire substrate in which a plurality of projections are formed onC-plane of the sapphire substrate by etching, comprising: forming apatterned etching mask on C-plane of the sapphire substrate; and etchingthe sapphire substrate until the projections are formed, wherein theprojections formed by the etching have a substantially pyramidal-shapeconstituted by a plurality of side surfaces to have a pointed top, theside surfaces having an inclined angle of between 53° and 59° from abottom surface of the projections.

Furthermore, the embodiment relates to a nitride semiconductor lightemitting element formed by growing a nitride semiconductor on theprincipal surface of any of the above-mentioned sapphire substrate.

With the sapphire substrate of the embodiment, a nitride semiconductorhaving excellent crystallinity can be grown, and a nitride semiconductorlight emitting element having excellent light extraction efficiency, inparticular light extraction efficiency from the side opposite to thesapphire substrate and from the direction perpendicular to the substratecan be obtained.

In addition, the method for manufacturing the sapphire substrate of theembodiment can provide a sapphire substrate on which a nitridesemiconductor having excellent crystallinity can be grown, and by usingthe sapphire substrate, a nitride semiconductor light emitting elementhaving excellent light extraction efficiency, in particular lightextraction efficiency from the side opposite to the sapphire substrateand from the direction perpendicular to the substrate can be obtained.

Furthermore, the nitride semiconductor light emitting element of theembodiment has the above-mentioned features and thus, consists ofnitride semiconductor having excellent crystallinity and has excellentlight extraction efficiency, in particular light extraction efficiencyfrom the side opposite to the sapphire substrate and from the directionperpendicular to the substrate.

[Sapphire Substrate]

FIG. 2 shows a cross section of a nitride semiconductor light emittingelement in one embodiment of the present invention. In FIG. 2, asemiconductor laminated structure 2 in which a base layer 21, a firstconductive layer (n-type layer) 22, an active layer (light emittinglayer) 23, a second conductive layer (p-type layer) 24 are laminated inthis order is formed on a sapphire substrate 10, and a plurality ofprojections having substantially pyramidal-shape with a pointed top,respectively, are formed on the surface of the substrate 10 on which thebase layer 21 is grown.

The sapphire substrate of embodiments of the present invention isprovided with a plurality of projections on one principal surface,wherein the projection has substantially a pyramidal-shape having apointed top and constituted by a plurality of side surfaces, wherein theside surface has an inclined angle of between 53° and 59° from a bottomsurface of the projection, and wherein the side surface is acrystal-growth-suppressed surface on which growth of nitridesemiconductor is suppressed relative to the substrate surface locatedbetween the adjacent projections.

In the present specification, “pointed top” means that there issubstantially no surface parallel to the substrate surface locatedbetween the adjacent projections, or the surface on the top of theprojection parallel to the substrate surface located between theadjacent projections is small enough not to allow a growth of nitridesemiconductor from the top of the projection.

In the present specification, “substantially pyramidal-shape” means ashape approximated by pyramidal-shape. Similarly, “substantiallytriangular pyramidal-shape”, “substantially polygonal shape”,“substantially triangular shape” and “substantially truncated triangularpyramidal-shape” mean shapes approximated by triangular pyramidal-shape,polygonal shape, triangular shape and truncated triangularpyramidal-shape, respectively. In particular, “substantiallypyramidal-shape (substantially triangular pyramidal-shape)” includes,for example, a shape in which each side of the bottom surface of thepyramidal-shape (triangular pyramid) has an outwardly curved arc shapeand each side surface is an outwardly curved rounded surface; a shape inwhich each side of the bottom surface of the pyramidal-shape (triangularpyramid) is a straight line and each side surface is flat; and a shapein which each side of the bottom surface of the pyramidal-shape(triangular pyramid) has an inwardly curved arc shape and each sidesurface is an inwardly curved rounded surface. In addition,“substantially polygonal shape (substantially triangular shape”includes, for example, a shape in which each side of the polygonal shape(triangular shape) has an outwardly curved arc shape, a shape in whicheach side of the polygonal shape (triangular shape) is a straight line,and a shape in which each side of the polygonal shape (triangular shape)has an inwardly curved arc shape.

Since the side surfaces constituting the projection have large inclinedangles of between 53° and 59° from the substrate surface located betweenthe adjacent projections, light propagating in the nitride semiconductorlight emitting element can be effectively reflected and/or diffracted onthe side surfaces of the projections and in particular, can be reflectedto a side opposite to the sapphire substrate in the directionperpendicular to the substrate. As a result, the light extractionefficiency especially from the side opposite to the sapphire substrateand from the direction perpendicular to the substrate can be improved.

Moreover, the sapphire substrate of embodiments of the present inventioncan achieve further improved light extraction efficiency by increasingthe height of the projections, and at the same time, it can achieve thegrowth of the nitride semiconductor having excellent crystallinity evenif the height of the projections are increased.

Hereinafter, it is described a mechanism how the nitride semiconductorhaving excellent crystallinity can be grown. In the sapphire substrateof embodiments of the present invention, each of the side surfacesconstituting the projection formed on the substrate is thecrystal-growth-suppressed surface on which the growth of the nitridesemiconductor is suppressed. On the other hand, the substrate surfacelocated between the adjacent projections is for example, C-plane, and isthe crystal growth surface on which the nitride semiconductor can begrown. When the nitride semiconductor is grown from the crystal growthsurface to the longitudinal direction (the direction perpendicular tothe sapphire substrate), crystal defects (dislocations) due to thedifference between a lattice constant of the substrate and that of thenitride semiconductor tend to extend to the growth direction andgenerate on the surface. When the nitride semiconductor is grown on thesapphire substrate of embodiments of the present invention, the nitridesemiconductor is grown from the crystal growth surface located betweenthe projections; however, the growth from the side surface of theprojection is suppressed, and the nitride semiconductor is notsubstantially grown. Therefore, the nitride semiconductor grown from thecrystal growth surface between the projections is grown to the lateraldirection on the side surface of the projection, and as the growthproceeds, the nitride semiconductor gets to cover the projections 1.This crystal growth to the lateral direction causes the crystal defectsextending to the growth direction to be trapped within the nitridesemiconductor, and as a result, crystal defects generated on the surfaceof the nitride semiconductor can be reduced. A light emitting elementstructure formed by laminating the nitride semiconductor layer havingreduced crystal defects can be obtained by further laminating thenitride semiconductor layer on the surface of this flat nitridesemiconductor having reduced crystal defects.

As illustrated in FIGS. 3A and 3B described below, the projection withrespect to embodiments of the present invention has substantiallypyramidal-shape with a pointed top, and has no crystal growth surface onthe top of the projection unlike the previous projection shown in FIGS.1A and 1B. Since the previous projection shown in FIGS. 1A and 1B has acrystal growth surface on the top of the projection, the growth of thenitride semiconductor in lateral direction as described above tends tobe impeded by the growth in longitudinal direction from the top of theprojection when the height of the projection is increased. On the otherhand, the projection with respect to embodiments of the presentinvention exhibit high effect of reducing crystal defects generated atthe surface of the nitride semiconductor since the growth of the nitridesemiconductor in lateral direction is not impeded by the growth inlongitudinal direction from the top of the projection even if the heightof the projection is increased.

Furthermore, the projection according to embodiments of the presentinvention can be arranged densely on the sapphire substrate because theprojections have substantially pyramidal-shape with a pointed top andthe inclined angles of the side surfaces constituting the projectionbeing between 53 and 59°. In case the inclined angles of the sidesurfaces are smaller, the height of the projections must be decreased ifthe projections are intended to be arranged densely. Embodiments of thepresent invention enable high-density arrangement of the projectionshaving its increased height on the sapphire substrate by increasing theinclined angles of the side surfaces constituting the projections. Aratio of the nitride semiconductor laterally growing so as to cover thecrystal-growth-suppressed surface is increased since the area ratio ofthe crystal-growth-suppressed surface to the crystal growth surface isincreased by arranging the projections densely, and crystal defectsextending in the growth direction are trapped within the nitridesemiconductor, and as a result, the crystal defects appearing on thesurface can be decreased. In addition, the sapphire substrate ofembodiments of the present invention can reduce a diffusion of incominglight by arranging the projections densely thereon, and the lightextraction efficiency of the obtained nitride semiconductor lightemitting element can be improved.

The bottom surface of the projection preferably has substantiallypolygonal shape having three or more sides which are outwardly curvedarc-shaped, respectively, and the side surfaces preferably havesubstantially triangular shape of which vertexes are both ends of a sideof the bottom surface and the top of the projection, respectively. Morepreferably, the bottom surface of the projection has substantiallytriangular shape. In particular, the side surfaces of the projectionsare preferably outwardly curved rounded surfaces.

FIGS. 3A and 3B show a projection according to one embodiment of thepresent invention. In this embodiment, the projection 1 hassubstantially triangular pyramidal-shape and its top t1 is pointed. Theprojection 1 is formed so as to have substantially triangularpyramidal-shape constituted by three side surfaces 11 k, 12 k and 13 k.The bottom surface of the projection has substantially triangular shapehaving sides 11, 12 and 13 having outwardly curved arc-shape, and theside surfaces 11 k, 12 k and 3 k are outwardly curved substantiallytriangular-shaped rounded surfaces, respectively. The three sidesurfaces 11 k, 12 k and 13 k of the projection 1 have the inclined angleof between 53˜59° from the substrate surface located between theadjacent projections, respectively. The inclined angle 14 of the sidesurface 11 k is shown in FIG. 3 (b) by way of example. As can be seenfrom FIG. 3B, the “inclined angle” of the side surfaces of theprojections means in the present specification, an angle between thebottom surface of the projection and a line connecting the top t1 of theprojection with a center of a side 11, 12 or 13 of the bottom surface ofthe projection.

As can be seen from FIG. 7B described below, the inclined angle of theprojections according to embodiments of the present invention issubstantially constant at any point on the line connecting the top ofthe projection with a center of a side of the bottom surface of theprojection. In other words, the side surfaces of the projections aresubstantially flat in a cross section passing through the top of theprojection and any of the centers of the sides of the bottom surface.

The height of the projections is preferably between 1.0 and 1.7 μm. Thenitride semiconductor light emitting element exhibiting excellent lightextraction efficiency can be obtained by setting the height of theprojections in this manner. In the present specification, “height ofprojection” means a distance between the top of the projection and thebottom surface of the projection.

The side surfaces constituting the projection arecrystal-growth-suppressed surfaces on which the crystal growth issuppressed compared to the crystal growth surface. Thecrystal-growth-suppressed surfaces provided on the sapphire substratefacilitate lateral growth of nitride semiconductor grown from thecrystal growth surfaces located on the adjacent projections to allowformation of nitride semiconductor having a surface with reduced crystaldefects. Thus, the projections having crystal-growth-suppressedsurface(s) are preferably distributed uniformly on the sapphiresubstrate and arranged to be apart from each other.

The projections are more preferably arranged periodically on thesapphire substrate. The projections are preferably arranged on eachvertex of triangular, tetragonal or hexagonal lattice. For example,specific arrangement of the projections includes arrangements such asparallelogram lattice-point arrangement and rectangular lattice-pointarrangement, other than the triangular lattice-point arrangement shownin FIG. 4 and square lattice-point arrangement shown in FIG. 5. Thetriangular lattice-point arrangement is particularly preferable sincepropagation in lateral direction of light generated in light emittinglayer can be effectively interrupted especially in case of high-densityarrangement. The projections may be formed with the same periodicarrangement on the entire surface of the sapphire substrate, oralternatively may be formed with different periodic arrangements on atleast two areas dividing the surface of the sapphire substrate dependingon the semiconductor element structure (for example, electrodearrangement) formed on the substrate.

A distance between the tops of the adjacent projections (hereinafterreferred to as a distance between the projections) is preferably between2.2 and 3.1 μm, and more preferably between 2.8 and 3.1 μm. By settingthe distance between the projections in this range, the crystallinity ofthe nitride semiconductor grown on the sapphire substrate as well as thelight extraction efficiency of the obtained nitride semiconductor lightemitting element are improved.

In the sapphire substrate of embodiments of the present invention, aratio of an area of the crystal growth surface to that of the principalsurface (hereinafter referred to as an area ratio of crystal growthsurface) is preferably between 25 and 50%, and more preferably between30 and 45%. A nitride semiconductor light emitting element exhibitinghigher light extraction efficiency can be obtained by setting the arearatio of crystal growth surface in this manner.

[Method for Manufacturing a Sapphire Substrate]

The method for manufacturing the sapphire substrate of embodiments ofthe present invention is characterized in that the plurality ofprojections are formed on C-plane of the sapphire substrate by etching,the method comprising an etching process of forming the plurality ofetching masks on C-plane of the sapphire substrate and etching thesapphire substrate until the projections become substantiallypyramidal-shaped with a pointed top.

Etching includes wet etching and dry etching, and embodiments of thepresent invention may employ any of the etching process.

As the dry etching, in particular, vapor-phase etching, plasma etchingand reactive ion etching can be used, and the etching gas in this regardincludes Cl-based and F-based gases such as Cl₂, SiCl₄, BCl₃, HBr, SF₆,CHF₃, C₄F₈, CF₄, for example, as well as inactive gases such as Ar. Themask material which can be used in dry etching includes oxide layerssuch as SiO₂ or metal layers such as Ni.

As an etching liquid for the wet etching, phosphoric acid orpyrophosphoric acid, or mixed acids obtained by adding them withsulfuric acid, or potassium hydroxide can be used. The mask material inthe wet etching can be selected depending on the substrate material andthe etching liquid used. For example, in addition to silicon oxides suchas SiO₂, an oxide(s) of at least one element selected from a groupconsisting of V, Zr, Nb, Hf, Ti, Ta and Al or a nitride(s) of at leastone element selected from a group consisting of Si, B and Al can beused.

One embodiment of the method for manufacturing the sapphire substrate ofembodiments of the present invention is described below with referenceto FIGS. 6A-6D.

Firstly, a plurality of circular-shaped etching masks 16 are formed byforming SiO₂ layer 15 and the like on C-plane (0001) of the sapphiresubstrate 10 (FIG. 6A) followed by patterning (FIG. 6B). The orientationof the sapphire substrate surface 10 on which the etching masks 16 areformed, the shape, dimension and arrangement of the etching masks 16,the distance between the etching masks 16 and the like can beappropriately selected depending on the targeted number, shape andarrangement of a plurality of the projections.

Then, the etching of the sapphire substrate 10 is performed. Uponbeginning the etching, a portion on which the etching masks 16 are notformed is removed by etching to form circular-shaped projections almostdirectly reflecting the shape of masks; however, the projections areaffected by an anisotropy in etching rate derived from crystalorientation (a difference in the propagation rate of etching betweendifferent directions) with the propagation of etching and thus, theprojections are formed into a shape reflecting the crystal orientation.

In particular, since the etching propagates with reflecting the crystalorientation due to the difference in etching rate between thepropagating direction of the etching, three vertexes of the bottom andridge lines of the triangular pyramid become gradually clear, so thatthe projections having substantially truncated triangularpyramidal-shape are formed under the circular-shaped etching masks 16.Upper surface of this substantially truncated triangularpyramidal-shaped projection is etched into substantially triangularshape and has an area less than the etching mask 16 due to an undercut.In addition, the bottom surface and the upper surface of the projectionare formed into substantially triangular shape consisting of therespective sides having outwardly curved arc shape, and as the etchingpropagates, the areas (of the bottom surface and the upper surface) isreduced and the radius of curvature of the respective sides is increasedto be rectilinear.

When the etching further propagates, the area of the upper surface ofthe projection becomes gradually decreased and at last, the projectionhas a shape such that the projection has a pointed top (FIG. 6C). Afterthe projection has substantially triangular pyramidal-shape having apointed top, the etching mask 16 is removed (FIG. 6D). Conditions forpromoting the undercut effectively may be employed. For example, when amixed acid comprising sulfuric acid and phosphoric acid is used as anetching solution, a ratio of phosphoric acid in the mixed acid may beincreased. Specific composition of the mixed acid can be adjusteddepending on a forming conditions of the etching mask and the like, andfor example, a mixed acid with a mixing ratio of (sulfuricacid):(phosphoric acid)=3:2 can be used. Also, the undercut may bepromoted effectively by reducing an adhesion between the etching maskand the sapphire substrate. The adhesion between the mask and thesubstrate can be reduced by adjusting the sputtering conditions of themask.

In this way, the plurality of the projections 1 can be formed on thesapphire substrate 10 so that the projections have substantiallypyramidal-shape with a pointed top and consist of the plurality of sidesurfaces, the side surfaces having the inclined angle of between 53 and59° from the bottom surface of the projection. The specific etchingconditions can be set depending on the shape of the targeted projectionsand the like.

It is also possible to form a projection 1 having triangularpyramidal-shape in which each side of the bottom surface is linear andeach side surface is flat by increasing the etching time. Also, it ispossible to form a projection 1 in which each side of the bottom surfacehas inwardly curved arc shape and each side surface is inwardly curvedrounded surface. Furthermore, it is also possible to form a projectionwith a bottom surface having quadrangular shape, pentagonal shape,hexagonal shape or the like by using dry etching.

By the above-mentioned one-stage etching step, projections can be formedwhich have a substantially pyramidal-shape (for example, substantiallytriangular pyramidal-shape) constituted by a plurality of side surfacesto have a pointed top, wherein the side surfaces have an inclined angleof between 53° and 59° from a bottom surface of the projections, andwherein the inclined angle is substantially constant at any point on aline connecting the top of the projection with a center of a side of thebottom surface of the projection.

By forming the projections by wet etching, the resulting projection hasa substantially triangular pyramidal-shape and the three side surfacesof this substantially triangular pyramidal-shape become close toR-plane. Therefore, the inclined angles of the respective side surfacesof the projection become close to the angle of R-plane. Specifically,the inclined angle is between 53° and 59°.

By the above-described method for manufacturing the sapphire substrate,the sapphire substrate which enables a growth of nitride semiconductorhaving excellent crystallinity can be obtained. By using the sapphiresubstrate, it can be obtained the nitride semiconductor light emittingelement exhibiting excellent light extraction efficiency, especiallyfrom the side opposite to the sapphire substrate and from the directionperpendicular to the substrate.

[Nitride Semiconductor Light Emitting Element]

A structure of a nitride semiconductor light emitting element has alaminated structure formed by laminating a first conductive layer 22, anactive layer 23, a second conductive layer 24 on the sapphire substrate10 in this order as shown in, for example, FIG. 2. A first electrode anda second electrode are formed on the first conductive layer 22 exposedby removing a part of the active layer 23 and the second conductivelayer 24 and on the second conductive layer 24, respectively, the secondelectrode consisting of a translucent ohmic electrode formed onapproximately the whole surface of the second conductive electrode 24,and a pad and a diffusion electrode formed on the translucent ohmicelectrode.

When the nitride semiconductor light emitting element of embodiments ofthe present invention is formed, gallium nitride-based compoundsemiconductor material represented by the general formulaIn_(x)Al_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1, 0≦x+y≦1) can be used as asemiconductor to be grown on the substrate, and in particular, itsbinary/ternary mixed crystal can be used preferably. Also, othersemiconductor materials than the nitride semiconductor, such as GaAs,GaP-based compound semiconductor, AlGaAs, InAlGaP-based compoundsemiconductor can be used.

The light emitting element obtained in this manner is constituted by thenitride semiconductor having excellent crystallinity and has excellentlight extraction efficiency, especially from the side opposite to thesapphire substrate and the direction perpendicular to the substrate.

EXAMPLE 1

Hereinafter, Examples with respect to embodiments of the presentinvention are described.

The projections were formed on a sapphire substrate by the followingprocedures.

A plurality of circular-shaped etching masks having diameters of about1.5 μm were formed on each vertex of triangular lattice with a side of1.9 μm by forming SiO₂ layer on a C-plane (0001) of the sapphiresubstrate and by patterning the SiO₂ layer. Subsequently, the substratewas immersed in an etching bath using a mixed acid of phosphoric acidand sulfuric acid as an etching liquid and etched with the solutiontemperature of about 290° C. for about 5 minutes. By this, a pluralityof projections having substantially triangular pyramidal-shape with apointed top were formed on the sapphire substrate. The inclined angle ofthe side surfaces constituting the projections was about 54°, the heightof the projections was about 1.2 μm, the distance between theprojections was 2.1 μm, and C-plane area ratio (area ratio of thecrystal growth surface) was 35%. SEM image from the upper surface of theobtained sapphire substrate and the cross sectional SEM image are shownin FIG. 7B and FIG. 7B, respectively.

The distances between the projections were changed in the range from 1.9to 3.1 μm, and for each of the distance between the projections, C-planearea ratios were changed in the range from 20 to 55% by adjusting thesize of the etching masks, the length of a side of the triangularlattice and the time of etching.

Subsequently, the substrate was transferred into MOCVD (metalorganicchemical vapor deposition) apparatus, and the base layer 21 having flatsurface was formed by growing GaN buffer layer of 20 nm at lowertemperature (about 510° C.) on the substrate surface on which theprojections were formed and on the buffer layer, growing along c-axisGaN at higher temperature (about 1050° C.).

On the base layer 21 obtained in this manner, the first conductive layer(n-type layer) 22 such as n-type contact layer, the active layer 23 andthe second conductive layer (p-type layer) 24 were laminated in thisorder as shown in FIG. 2.

In particular, the n-type contact layer of Si (4.5×10¹⁸/cm³)-doped GaNhaving thickness of 5 μm is formed on the base layer 21 as the firstconductive layer (n-type layer) 22. Further, in the region between then-type contact layer and the active layer, an undoped GaN layer of 0.3μm, a Si (4.5×10¹⁸/cm³)-doped GaN layer of 0.03 μm, an undoped GaN layerof 5 nm, a multilayer formed by repeatedly and alternatively laminatingundoped In_(0.1)Ga_(0.9)N layers of 2 nm and undoped GaN layers of 4 nmby 10 for each layer and at last laminating an undoped In_(0.1)Ga_(0.9)Nlayer of 2 nm were laminated as the first conductive layer (n-typelayer) 22. Then, a multiple quantum well structure formed by repeatedlyand alternatively laminating undoped GaN barrier layers having thicknessof 25 nm and In_(0.3)Ga_(0.7)N well layers having thickness of 3 nm by 6for each layer and at last laminating a barrier layer was laminated asthe active layer 23 on the n-type layer 22. Then, a p-side multilayerformed by repeatedly and alternatively laminating Mg (5×10¹⁹/cm³)-dopedAl_(0.15)Ga_(0.85)N layers having thickness of 4 nm and Mg(5×10¹⁹/cm³)-doped In_(0.03)Ga_(0.97)N layers having thickness of 2.5 nmby 5 for each layer and at last laminating the above-mentioned AlGaNlayer and a p-type contact layer of Mg (1×10²⁰/cm³)-doped GaN havingthickness of 0.12 μm were laminated as the second conductive layer(p-type layer) 24 on the active layer 23.

A part of the n-type layer 22 was exposed by removing a part of thenitride semiconductor layer from the p-type layer 24 by means ofetching. The first electrode and the second electrode were formed on theexposed n-type layer 22 and on the p-type layer 24, respectively. Inparticular, ITO (about 170 nm) was formed as the translucent ohmicelectrode on the surface of the p-type layer 24, which is the surface ofthe light emitting structure portion, and the electrode having astructure formed by laminating Rh (about 100 nm), Pt (about 200 nm) andAu (about 500 nm) in this order was formed on this ITO and on the n-typecontact layer. In this manner, the nitride semiconductor light emittingelement exhibiting an emission wavelength of 465 nm was manufactured.

COMPARATIVE EXAMPLE 1

Then, nitride semiconductor light emitting elements were prepared whichwere obtained by growing nitride semiconductor on sapphire substrateshaving substantially truncated triangular pyramidal-shaped projectionsformed thereon.

Firstly, projections having substantially truncated triangularpyramidal-shape were formed on C-plane of a sapphire substrate byprocedures shown below.

A plurality of circular-shaped etching masks having diameters of about1.5 μm were formed on each vertex of triangular lattice with a side of2.6 μm by forming SiO₂ layer on a C-plane (0001) of the sapphiresubstrate and by patterning the SiO₂ layer. Subsequently, the substratewas immersed in an etching bath using a mixed acid of phosphoric acidand sulfuric acid as an etching liquid and etched with the solutiontemperature of about 290° C. for about 4.5 minutes. By this, a pluralityof projections having flat surfaces on the top were formed on thesapphire substrate. The inclined angle of the side surfaces constitutingthe projections was between about 53 and 59°, the height of theprojections was about 1 μm, the distance between the projections was 3.5μm, and C-plane area ratio was 60%. SEM image from the upper surface ofthe obtained sapphire substrate is shown in FIG. 8.

The distances between the adjacent projections were changed in the rangefrom 1.9 to 3.1 μm, and for each of the distance between theprojections, C-plane area ratios were changed in the range from 20 to55% by adjusting the size of the etching masks, the length of a side ofthe triangular lattice and the time of etching as the above-describedExample. In Comparative Examples, “distance between projections” means adistance between the centers of the bottom surface of the adjacentprojections. Although C-plane area ratio in Comparative Examples isfollowed by the definition of crystal growth surface area ratio asdescribed above, C-plane area in Comparative Examples is defined by thesum of the area of C-plane located between the adjacent projections andthe area of flat surfaces formed on the top of projections.

Using the substrates obtained in this manner, nitride semiconductorlight emitting elements of Comparative Example 1 were prepared by thesimilar procedures to those of the above-described Example 1.

[Measurement of Luminous Flux]

Bullet-shaped light emitting devices were prepared using the obtainedlight emitting elements of Example 1 and Comparative Example 1. In thebullet-shaped light emitting device, the light emitting element isplaced on a first lead, and the first lead and a second lead areelectrically connected to the light emitting element via wires. Thefirst lead does not have a cup shape, and the light emitting device isplaced on a flat surface of the first lead. A part of the first lead anda part of the second lead as well as the light emitting element iscovered with epoxy resin and immobilized. A part of the epoxy resinhemispherical has hemispherical shape such that the light emittingelement placed on the first lead is at the center.

Each of the prepared light emitting devices of Example 1 and ComparativeExample 1 was placed at the center of a spherical integrating sphere,and the luminous flux Φ_(v) was determined.

For the light emitting devices of Example 1 and Comparative Example 1which have the same distance between the projections and the C-planearea ratio, luminous flux ratio (Φ_(v) ratio) of the light emittingdevice of Example 1 to that of Comparative Example 1 is defined by thefollowing formula (1).

$\begin{matrix}{{{Luminous}\mspace{14mu}{flux}\mspace{14mu}{ratio}\mspace{14mu}\left( {\phi_{v}\mspace{14mu}{ratio}} \right)\;\left( {a.u.} \right)} = \frac{\begin{matrix}{{{Luminous}\mspace{14mu}{flux}\mspace{14mu}{of}\mspace{14mu}{light}}\mspace{11mu}} \\{{emitting}\mspace{14mu}{device}\mspace{14mu}{of}\mspace{14mu}{Example}}\end{matrix}\;}{\begin{matrix}{{{Luminous}\mspace{14mu}{flux}\mspace{14mu}{of}\mspace{14mu}{light}\mspace{14mu}{emittng}}\mspace{11mu}} \\{{device}\mspace{14mu}{of}\mspace{14mu}{Comparative}\mspace{14mu}{Example}}\end{matrix}\;}} & \left\lbrack {{For}\;{mula}\mspace{14mu} 1} \right\rbrack\end{matrix}$For each of the light emitting devices of Example 1 and ComparativeExample 1 which have the same distance between the projections and theC-plane area ratio, the amount of luminous flux ratio was calculatedusing the formula (1). FIG. 9 shows a graph plotting the obtainedluminous flux to the C-plane area ratio.

As can be seen from FIG. 9, the luminous flux ratio (Φ_(v) ratio) of thelight emitting devices of Example 1 to those of Comparative Exampletends to be more than 1. By this, the light emitting devices of Example1 were found to exhibit increased luminous flux and thus, improved lightextraction efficiency compared to the light emitting device ofComparative Example 1. The luminous flux ratio tends to be increasedwith the increased distance between the projections, and particularlylarge value of the luminous flux ratio was obtained when the distancebetween the projections was between 2.2 and 3.1 μm, and in particularbetween 2.8 and 3.1 μm. In addition, for any distances between theprojection, particularly large value of luminous flux ratio was obtainedwhen C-plane area ratio was between 25 and 50%, and in particularbetween 30 and 45%.

EXAMPLE 2

Then, nitride semiconductor light emitting elements of Example 2 andComparative Example 2 were prepared which had the distance between theprojections of between 1.9 and 3.1 μm and C-plane area ratio of betweenabout 35% and 70% by the similar procedures to those explained inExample 1 and Comparative Example 1 described above. These nitridesemiconductor light emitting elements of Example 2 and ComparativeExample 2 have the same constitution as those of Example 1 andComparative Example 1 described above except that the distance betweenthe projections was between 1.9 and 3.1 μm and C-plane area ratio wasbetween about 35% and 70%.

[Measurement of Luminous Flux]

Bullet-shaped light emitting devices were prepared using the obtainedlight emitting elements of Example 2 and Comparative Example 2,similarly to Example 1 and Comparative Example 1 described above. Eachof the prepared light emitting devices of Example 2 and ComparativeExample 2 was placed at the center of a spherical integrating sphere,and the radiant flux Φ_(e) and the luminous flux Φ_(v) in all directionswere determined.

For the nitride semiconductor light emitting elements of Example 2 andComparative Example 2 which had the same distance between theprojections and C-plane area ratio, the luminous flux ratio (Φ_(v)ratio) of the nitride semiconductor light emitting element of Example 2to that of Comparative Example 2 is defined by the formula (1) describedabove, and the radiant flux ratio (Φ_(e) ratio) of the nitridesemiconductor light emitting element of Example 2 to that of ComparativeExample 2 is defined by the following formula (2).

$\begin{matrix}{{{Radiant}\mspace{14mu}{flux}\mspace{14mu}{ratio}\mspace{14mu}\left( {\phi_{e}\mspace{14mu}{ratio}} \right)\;\left( {a.u.} \right)} = \frac{\begin{matrix}{{{Radiant}\mspace{14mu}{flux}\mspace{14mu}{of}\mspace{14mu}{light}}\mspace{11mu}} \\{{emitting}\mspace{14mu}{device}\mspace{14mu}{of}\mspace{14mu}{Example}}\end{matrix}\;}{\begin{matrix}{{{Radiant}\mspace{14mu}{flux}\mspace{14mu}{of}\mspace{14mu}{light}\mspace{14mu}{emittng}}\mspace{11mu}} \\{{device}\mspace{14mu}{of}\mspace{14mu}{Comparative}\mspace{14mu}{Example}}\end{matrix}\;}} & \left\lbrack {{For}\;{mula}\mspace{14mu} 2} \right\rbrack\end{matrix}$The luminous flux ratios and the radiant flux ratios were calculatedusing the formulas (1) and (2) for each of the light emitting devices ofExample 2 and Comparative Example 2. FIG. 10 shows a graph plotting theobtained radiant flux ratios and the luminous flux ratios to the C-planearea ratio.

As can be seen from FIG. 10, radiant flux ratio (Φ_(e) ratio) andluminous flux ratio (Φ_(v) ratio) both tend to be more than 1 whenC-plane area ratio is about 60% or less, and particularly increasedradiant flux ratio and luminous flux ratio were achieved when C-planearea ratio was between 25 and 50%, particularly between 30 and 45%. Bythis, it is found that the light emitting device of Example 2 accordingto the present invention has increased luminous flux compared to thelight emitting device of Comparative Example 2, and thus, has improvedlight extraction efficiency.

In addition, as can be seen from FIG. 10, the value of the luminous fluxratio tends to be larger than that of the radiant flux ratio when theC-plane area ratio is in the range between about 35% and 70%. Thistendency was significant when the C-plane area ratio was between 25 and50%, in particular between 30 and 45%. By this, the light emittingdevice of Example 2 was found to have excellent light emittingefficiency compared to the light emitting device of Comparative Example2.

EXAMPLE 3

A nitride semiconductor light emitting elements of Example 3 which hadthe distance between the projections of 2.5 μm and C-plane area ratio of51% was prepared by the similar procedures to those of theabove-described Example 1. This nitride semiconductor light emittingelements of Example 3 had the same configuration as that of Example 1except that the distance between the projections was 2.5 μm and C-planearea ratio was 51%.

COMPARATIVE EXAMPLE 3

As a nitride semiconductor light emitting element of Comparative Example3, a nitride semiconductor light emitting element was prepared which wasobtained by growing a nitride semiconductor on a sapphire substrate,wherein the projections having a substantially triangularpyramidal-shape were formed, and wherein the inclined angle of sidesurfaces of the projections varies in two stage.

Firstly, projections were formed on C-plane of a sapphire substrate byprocedures shown below.

A plurality of circular-shaped etching masks having diameters of about1.5 μm were formed on each vertex of triangular lattice with a side of2.5 μm by forming SiO₂ layer on a C-plane (0001) of the sapphiresubstrate and by patterning the SiO₂ layer. Subsequently, the substratewas immersed in an etching bath using a mixed acid with reduced mixingratio of phosphoric acid compared to the mixed acid used in Example 1(specifically, sulfuric acid:phosphoric acid=3:1) as an etching liquidand etched with the solution temperature of about 290° C. for about 4.5minutes. And then, the etching masks were removed, the substrate wasimmersed again in the etching bath described above and etched with thesolution temperature of about 290° C. for about 1 minute. By this,substantially triangular pyramidal-shaped projections in which theinclined angle of the side surface varies in two stage were formed onthe substrate. The inclined angle of the side surfaces constituting theprojections was 53° on the lower side and 28° on the upper side. Theheight of the projections was about 1 μm, the distance between theprojections was 2.5 μm, and C-plane area ratio was 38%. Cross-sectionalSEM image of the obtained sapphire substrate is shown in FIG. 11. FromFIG. 11, it is found that the inclined angle of the side surface variesin two stage at the cross-section passing through the top of theprojection. Using the substrate obtained in this manner, a nitridesemiconductor light emitting element of Comparative Example 3 wasprepared by similar procedures to those of Example 1 described above.

For each of the nitride semiconductor light emitting elements of Example3 and Comparative Example 3 obtained in this manner, the lightintensities were measured and the directional characteristics wereevaluated. The results are shown in FIG. 12. FIG. 12A is a top view ofthe nitride semiconductor light emitting elements of Example 3 andComparative Example 3. FIG. 12B is a graph showing the directionalcharacteristics of the nitride semiconductor light emitting elements ofExample 3 and Comparative Example 3 when the light intensities weremeasured in a direction of φ=0° shown in FIG. 12A, and FIG. 12C is agraph showing the directional characteristics of the nitridesemiconductor light emitting elements of Example 3 and ComparativeExample 3 when the light intensities were measured in a direction ofφ=90° shown in FIG. 12A. In FIG. 12B and FIG. 12C, the radial directionindicates the light intensity and the circumferential directionindicates the directivity angle. The light intensities are shown asrelative values based on the maximum light intensity in each of thenitride semiconductor light emitting elements.

From FIGS. 12A, 12B, and 12C, it was found that the nitridesemiconductor light emitting element of Example 3 in which theprojections have the constant inclined angle exhibited improved lightextraction efficiency to the front direction of the nitridesemiconductor light emitting element, compared to the nitridesemiconductor light emitting element of Comparative Example 3 in whichthe projections have the inclined angle varying in two stage, in anycase of φ=0° and φ=90°.

The sapphire substrate of embodiments of the present invention canprovide a nitride semiconductor light emitting element having highoutput and light extraction efficiency.

What is claimed is:
 1. A sapphire substrate provided with a plurality ofprojections on a principal surface on which a nitride semiconductor isgrown to form a nitride semiconductor light emitting element, whereinthe projections have a substantially triangular pyramidal-shape theprojections having a plurality of side surfaces and a pointed top,wherein the side surfaces have an inclination angle of between 53° and59° from a bottom of the projections, and wherein the side surfaces arecrystal-growth-suppressed surfaces on which a growth of the nitridesemiconductor is suppressed relative to a portion of the principalsurface located between adjacent projections, wherein a bottom of theprojections has a substantially triangular shape having three outwardlycurved arc-shaped sides, and each of the side surfaces has asubstantially triangular shape having vertexes located at the top of theprojection and at both ends of a respective side of the bottom of theprojection, and wherein the projections are arranged on vertexes of atriangular lattice, and an orientation of the bottom of said projectionsconforms with an orientation that is rotated by about 30 degrees from anorientation of a triangle of the triangular lattice.
 2. The sapphiresubstrate according to claim 1, wherein the projections have a height ofbetween 1.0 and 1.7 μm.
 3. The sapphire substrate according to claim 1,wherein the projections are separated from each other on the principalsurface of the sapphire substrate.
 4. The sapphire substrate accordingto claim 1, wherein the projections are arranged periodically on theprincipal surface of the sapphire substrate.
 5. The sapphire substrateaccording to claim 1, wherein a distance between tops of the adjacentprojections is between 2.2 μm and 3.1 μm.
 6. The sapphire substrateaccording to claim 5, wherein the distance between tops of the adjacentprojections is between 2.8 μm and 3.1 μm.
 7. The sapphire substrateaccording to claim 1, wherein the portion of the principal surfacelocated between the adjacent projections is a crystal growth surface,and wherein a ratio of an area of the crystal growth surface to an areaof the principal surface is between 25% and 60%.
 8. The sapphiresubstrate according to claim 7, wherein the ratio of the area of thecrystal growth surface to that of the principal surface is between 30%and 45%.